Heat storage device having a high volume capacity

ABSTRACT

The invention is a heat storage device having a high volume capacity (defined as the ratio between the volume of the heat storage medium divided by the overall volume of the device) combined with a high heat transfer surface and a low envelope material volume (‘dead volume’). The heat storage device is composed of individual storage elements which encapsulate a heat storage medium. The storage elements are stacked in a way that creates channels for the passage of a heat transfer fluid. The (air) heat transfer fluid channels are surrounded by a large heat transfer surface and embossed in a way to improve the heat transfer.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a heat storage device.

2. Description of the Related Art

Heat storage technology is well known for storing heat from a fluid into a storage medium. A heat storage device takes heat from a heat transfer fluid and stores it into a heat storage medium. An illustrative example is a system to store heat from hot air into a volume of water. In a typical design, heat transfer fluid, such as hot air, is forced through layers of metallic heat storage elements folded in a zig-zap pattern, similar in shape to a accordion bellows. The heat storage elements are typically formed from two plates of metal with a heat storage medium between the plates. The performance of a heat storage device is dependent on two important aspects, the heat exchange between a heat transfer fluid and the plates, and the heat transfer inside the heat storage medium. The state of the art for this type of system typically uses metallic plates for transferring heat from a heat transfer fluid, and a heat storage medium with a high thermal conductivity for efficient transfer of heat inside of itself.

In WO2011153595, Prieels discloses a counter flow heat exchanger body having a accordion shape and comprising substantially planar transverse heat-exchange walls. Prieels discloses the body exchange walls are made of paper, coated with a protective layer of plastic.

It is found that both metallic and coated paper materials have a limited ability to transfer the heat of a transfer fluid to a heat storage medium. It is also found that typical heat storage devices develop a pressure drop from the input of the system to the output, which reduces the efficiency of heat transfer. It is also found that heat transfer is improved with a higher ratio of surface area of the plates to the volume of heat storage medium, and a better convection coefficient of the heat storage medium. It is also found that typical heat storage devices experience corrosion of the plates and housings due to the typically harsh conditions to which they are subject.

There is a need for a heat storage device with higher storage density, less pressure drop, and a higher power of charging and discharging the storage system (higher speed of exchange). There are additional needs for a heat storage device that is corrosion-proof, yet has economical material and production costs.

SUMMARY OF THE INVENTION

One embodiment of the invention is a heat storage device having a high volume capacity (defined as the ratio between the volume of the heat storage medium divided by the overall volume of the device) combined with a high heat transfer surface and a low envelope material volume (‘dead volume’). The heat storage device is composed of individual elements encapsulating the heat storage medium stacked in a way that creates fluid transfer channels. The (air) heat transfer fluid channels are surrounded by a large heat transfer surface and embossed in a way to improve the heat transfer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows an embossed sheet, and a pair of embossed sheets forming a storage element.

FIG. 2 shows various features of a storage element.

FIG. 3 shows two storage elements stacked to form channels.

FIG. 4 shows a top view of a storage element.

FIG. 5 shows a storage module.

FIG. 6 shows storage modules stacked in parallel.

FIG. 7 shows storage modules stacked in series.

FIG. 8 shows storage modules stacked in parallel and in series.

DEFINITIONS

As employed herein, the term “storage element” means a thin material encapsulating a heat storage medium.

As employed herein, the term “storage module” means an assembly of two or more storage elements, with air channels in between.

As employed herein, the term “storage device” means an assembly of two or more storage modules, connected in serial or parallel.

As employed herein, the term “heat storage medium” means any material with storage capacity for sensible or latent heat.

As employed herein, the term “heat transfer fluid” means any gas, such as, as air, inert gas, or helium; or liquid, such as water, glycol-water, or oil.

As employed herein, the term “compactness” means the ratio between the surface area of a storage element, and the volume contained by that surface area.

As employed herein, the term “volume capacity” means the ratio between the volume of the heat storage medium divided by the overall volume of the storage device.

As employed herein, the term “PCM” means a phase change material.

As employed herein, the term “mean airflow velocity” means the overall volumetric airflow rate divided by the frontal entering area of all the air channels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a heat storage device that is inexpensive and corrosion-proof, while ensuring an excellent heat transfer between the heat transfer fluid and the heat storage medium. The invention relates to a heat storage device for the transfer of the heat energy of a fluid into a heat storage medium. The heat storage device is formed by assembling storage elements into storage modules, and storage modules into the storage device.

The storage elements consist of sheets of thermally conductive materials that are formed with a particular shape, and assembled in such a manner, that a large ratio of volume to surface area is created. The storage elements are stacked in a way to create air channels in between each for the heat transfer fluid. Heretofore, designs for heat storage elements are based on optimizing the thermal conductivity of the material enveloping the storage material. The design of the storage element of the present invention is based on the optimization of the heat transfer in and out of the storage material. It depends from one side from the area size and the topography of the exchange surface and from the other side from the storage element thickness.

The optimum thickness of the thermally conductive material from which the storage element is constructed is calculated by taking into consideration the storage performance of the heat storage medium, and the required time for cycling the charge and discharge of the heat storage device, based on the depth of penetration of heat into the heat storage medium.

Sigma=SQRT((lambda*T)/(pi*Rho*Cp)).

Wherein lambda=thermal conductivity, T=cycle period , Rho=density, and Cp=average heat capacity over the given temperature interval (include sensible+latent heat).

This approach is based by solving the heat transfer equation for a harmonic source as described in ISO 13786 “Thermal performance of building components —Dynamic thermal characteristics—Calculation methods”, available from the International Organization for Standardization.

The storage elements are formed by embossing parallelepiped shapes into polymeric sheet materials using thermoforming methods well known in the art of polymer processing. Two nesting shapes connected by a hinge portion are embossed in each thermoforming operation. The two embossed portions fold along the hinge and nest together in a clamshell fashion to form the series of small interconnected volumes that is subsequently filled with a heat storage media, to create the storage element. The embossing pattern of the storage elements is optimized to minimize the pressure drop across the storage device by the cross corrugation pattern design of the heat transfer channels. The cross corrugation pattern embossed in the plate induce the heat storage fluid to be embedded in a multitude of small volumes with a characteristic distance equal to Sigma, with each small volume surrounded by the heat transfer fluid.

Two or more storage elements are stacked to form a storage module. The two or more storage elements in the storage module are stacked in a way that creates channels between the elements for passage of heat transfer fluid. The channels are designed in a way to have alternating sections with an aspect ratio changing alternatively from height to width, to width to height, wherein the height is greater in dimension than the width.

The volume capacity of the storage element is calculated by multiplying the overall thickness times the surface area, to which the volume occupied by the heat transfer fluid is subtracted. It has been found that minimizing the diameter of the channels will improve the heat transfer coefficient and increase the volume capacity.

The energy storage capacity of a single element is calculated by the integration of the sensible and latent heat of the heat storage medium on a defined temperature range time the overall heat storage medium mass contained in the element. It can be optimized by either increasing the sensible or latent heat of the heat storage medium or the overall mass of the element. 

What is claimed is:
 1. A heat storage element consisting of an embossed polymeric sheet encapsulating a heat storage medium.
 2. A heat storage element as described in claim 1, where the thickness of the embossed polymeric sheet is in the range from 0.1 to 1 mm.
 3. A heat storage device comprised of two or more of the heat storage elements of claim 1 stacked in a way that creates fluid transfer channels.
 4. A heat storage device as described in claim 3 and having a volume capacity ranges between 60% and 90%.
 5. A heat storage device as described in claim 3 and having a heat transfer compactness of 212 m²/m³.
 6. A heat storage device as described in claim 3 wherein the embossed polymeric sheet is made of a polymeric material selected from PVC, ABS, PP, HDPE, or PS.
 7. A heat storage device as described in claim 3, wherein the heat storage medium can be any material having a high sensible and latent heat, and having a melting temperature between −40 deg C and 150 deg C.
 8. A heat storage device as described in claim 3 wherein the heat storage density is greater than 5 Wh/liter, on a 15° C. temperature difference.
 9. A heat storage device as described in claim 3 wherein the thickness of a storage element is between 5 and 25 mm.
 10. A heat storage device as described in claim 3 wherein the pressure drop through a module is less than 50 Pa, with a mean airflow velocity higher than 1 m/s.
 11. A heat storage device as described in claim 3 where the overall heat coefficient of exchange is higher than 12 W/m²/K with an mean airflow velocity less than 2 m/s. 